Morphology

La0.5Sr0.5Mn0.5Co0.5O3-δ (LSMCo) and La0.9Sr0.1Mn0.5Cr0.5O3-δ (LSMCr) are not reported due to their poor electrochemical performance.

4.1. Morphology

In USP method, a precursor solution is atomized into droplets by a piezoelectric transducer and continuously carried into a hot reaction zone. The precursor droplets undergo a series of stages within the pyrolysis zone, i.e., solvent evaporation, solute precipitation, decomposition, and sintering (Figure 4-1a). The final particle size of the product essentially depends on the generated precursor droplet size, since the concept of the synthesis method relies on the fact that one droplet forms one product particle [201]. The SEM images of the as-synthesized powders of LSCand NiO-GDC20show the typical hollow sphere morphology of ultrasonic spray pyrolysis (Figure 4-1b–c).

Such shell-like particle morphology is observed when the precursor concentrations are relatively low. The evaporation of the solvent from the droplet surface occurs at a rate faster than the diffusion of the solute, which leads to an increase of the solute concentration near the surface of the droplets. Above the critical super-saturation concentration, the solute starts to precipitate and decompose on the surface of the droplets, thereby resulting in a shell-like particle formation [202].

The SEM images in Figure 4-1b and c indicate that most of the particles retain their hollow sphere morphology and only a small fraction of the particles have broken shells. The conservation of the shell-like particle morphology can be explained by the high solvent permeability of the precipitate shells. If the precipitate shells are sufficiently permeable, they can be preserved as hollow spheres during the evaporation of the solvent from the droplet [201]. In addition, it is observed that the particle sizes are broadly distributed over a range of 0.1–3.0 m. The broad particle size distribution of spray pyrolyzed powders is often associated with the high density of droplets in the

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Figure 4-1: a) Illustration of the particle formation in an ultrasonic spray pyrolysis process. SEM images of b) LSC and c) NiO-GDC20 powders synthesized at 775 °C.

aerosol phase as the droplet collisions and subsequent coalescence lead to the formation of larger secondary droplets within the pyrolysis zone. It is also noteworthy to mention that the hollow spheres are essentially nanoporous due to the porosity between precipitated solute nanocrystallites. However, they are virtually inseparable in most cases as they form strongly agglomerated and/or sintered three-dimensional networks [203].

It has been recently reported that the incorporation of non-volatile, bystander inorganic salts with the precursor solutions and subsequent water rinsing of ultrasonic spray pyrolyzed product lead to the fragmentation of the polycrystalline microspheres into much smaller nanoparticles [204]. This modified synthesis route is known as salt-assisted spray pyrolysis (SASP). Figure 4-2a illustrates the particle formation process of SASP. This novel technique depends on the distribution of salt on the surfaces of crystallites during the pyrolysis stage of the synthesis, which effectively inhibits the agglomeration and sintering of the primary particles emerged from the solute decomposition. The SEM images of salt-assisted spray pyrolyzed LSC and NiO-GDC20 powders (Figure 4-2b–c) show substantial difference in terms of morphology, particle size and distribution compared to the powders produced by conventional spray pyrolysis method (Figure 4-1b–c), while all the other synthesis parameters are kept unchanged. Instead of the hollow sphere morphology, both LSC and NiO-GDC20powders consist of well-dispersed nanoparticles with a low degree of agglomeration and the particle sizes are distributed over a narrow range of 25–75 nm. For both material systems, the modification of precursor by NaCl leads to particle size reduction by a factor of 20–40. It is also observed that the powders have significantly sharpened particle size distributions compared to the USP derived powders. In contrast to the USP method, the size distribution of particles obtained by SASP is apparently independent of the size distribution of the precursor droplets, which can be explained by the effective separation of precipitated solute nanocrystallites within the droplets by the NaCl phase. Various morphological characteristics of

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Figure 4-2: a) Illustration of particle formation in a salt-assisted spray pyrolysis process. SEM images of b) LSC and c) NiO-GDC20 powders synthesized at 775 °C (after water rinsing).

nanoparticles can be obtained by changing the process parameters within the SASP. Therefore, the effects of the salt concentration and the synthesis temperature on the morphology are studied using LSC nanoparticles as a model system.

4.1.1. Effect of NaCl Concentration on Morphology

The SEM images in the Figure 4-3 illustrate the morphological evolution of LSC powders synthesized at 775 °C as the concentration of NaCl is changed from 0 to 1 M. The synthesis pressure and the total cation concentration of La, Sr, and Co are kept constant at 900 mbar and 0.05 M, respectively, for each synthesis. SEM images of the powders obtained by SASP are taken subsequent to the water rinsing. The SEM image Figure 4-3a shows that the LSC powder synthesized without NaCl consists of spherical hollow particles with diameters in the range of 0.1–

3 m. On the other hand, when the precursor solution contains NaCl, it is observed that the particle sizes of LSC powders are reduced predominantly down to the nanometer scale. At relatively low concentrations of NaCl (0.25 M), the primary LSC nanoparticles with approximate particle size of 50 nm form agglomerated secondary particles in micrometer scale. This is most probably due to the insufficient amount of inert salt phase, which cannot hinder the agglomeration and sintering of the primary LSC nanoparticles, as it is evident from Figure 4-3b. The degree of agglomeration is reduced substantially, as the concentration of NaCl increases to 0.5 M.

Nevertheless, flake-like agglomerates with sizes around 250 nm are observed in SEM images

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Figure 4-3: SEM images of LSC particles synthesized at 775 °C with various NaCl concentration of a) 0 M, b) 0.25 M, c) 0.5 M, d) 1.0 M. The SEM images of the particles in b, c, and d are taken after water rinsing.

(Figure 4-3c). At the salt concentration of 1.0 M, a complete fragmentation of the LSC powder is accomplished (Figure 4-3d) subsequent to the water rinsing of the as-synthesized product.

4.1.2. Effect of Synthesis Temperature on Morphology

To investigate the influence of the pyrolysis temperature on the morphology of the product, LSC nanoparticles are synthesized at different temperatures. The synthesis pressure and the total cation concentration of La, Sr, and Co are kept constant at 900 mbar and 0.05 M, respectively, for each synthesis. Figure 4-4 shows the SEM images of the powders synthesized at 700 °C, 775 °C, 900 °C, and 1000 °C with the NaCl concentration fixed to 1.0 M. As it is evident from the SEM images in Figure 4-4a and b, LSC nanoparticles with a low degree of agglomeration and an approximate primary particle size of 50 nm can be obtained at synthesis temperatures below 800 °C. However, a considerable amount of agglomerates in the size range of 0.2–1 m are observed in the LSC powders synthesized at 900 °C (Figure 4-4c). As the synthesis temperature reaches to 1000 °C, the degree of agglomeration continues to increase. Moreover, it is observed that the primary particles of LSC sinter to form dense particles in the size range of 0.5–1 m (Figure 4-4d), since crystal growth and molecular diffusion processes are fast at increased synthesis temperatures. Even though the primary LSC nanoparticles are surrounded by sufficient amount of NaCl, when the salt phase is melted (m.p. = 801 °C), it behaves as a molten solvent and eventually facilitates the sintering and densification processes of the primary particles. In this case, the LSC agglomerates

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Figure 4-4: SEM images of LSC particles at different synthesis temperatures: a) 700 °C b) 775 °C, c) 900 °C, d) 1000 °C. The SEM images of the particles are taken after washing with water.

can be self-sintered without any obstacle. On the other hand, at synthesis temperatures below melting point of the NaCl the sintering of the LSC nanoparticles can be prevented because of the effective covering of the nanoparticle surfaces with the non-molten salt phase.